Hop Diseases and Pests

Introduction

This information is intended to help identify common problems associated with growing hops. If detection is made in the early stages of growth, there is a better chance of developing a solution and reducing the effects of the problem on your harvest and increasing yield.

Please note that most of the pests and diseases have humuli in the Latin name. This means that they are specific problems on hops and do not infect or inhabit other plants. Therefore, if hops do not have a history of growing near your location, these problems will hopefully not exist in your area. Don't let the potential problems of growing hops stop you anymore than the potential of brewing a bad batch of beer. Mainly because of the higher heat used in drying commercial hops, the full aromatic potential may be somewhat diminished. Therefore, by using lower drying temperatures and hopefully organic growing conditions, homegrown hops are the best.

Good luck in your growing efforts! —Freshops

Nutrient Deficiency Symptoms

Major nutrient deficiency symptoms of NPK

These can be corrected by simply fertilizing the plants. Unhealthy plants become susceptible to attack by disease and insects. The first steps in reducing problems is the initial soil preparation, nutrient supplies and watering.

Nitrogen Deficiency- Leaves are smaller, dark olive green and have a dull appearance. They can fade to a dull orange. There is a tendency for the leaves to curl downward and the leaf petioles to become darker red. Brown spots develop interveinally on the under surface of the leaves.

Potassium Deficiency- The first symptoms are downward curling of the leaf, associated with a bronzing of the interveinal tissue. The interveinal tissue becomes a paler green and eventually dies, giving rise to interveinal scorch. In the final stages, the leaves wither to an ashy-gray color and fall. There may be a marginal scorch on the older leaves.

Hop Insect Pests

Insects can generate large numbers very quickly. Your best defense is to check your plants daily or even twice daily, morning and evening, especially in the early stages of growth. Problems can be corrected and damage reduced if caught early.

Aphids

Hop Aphid (Phorodon humuli)

A frequent pest on hops. It passes the winter as an egg on woody hosts of the genus Prunus (cherry, peach, plum). Winged aphids move to the top of the plant late in the spring. Aphid populations may build very rapidly and if left uncontrolled, may result in defoliation.

Aphid Generations

Sometimes mother aphids carry embryos that are carrying their own embryos. This telescoping reproduction strategy results in quick population growth.

Hop Insect Pests

Spider Mites

Two-spotted Spider Mite (Tetranychus urticae)

Mobile Adult (#1)

A serious pest of hops grown in the warm interior valleys of the northwestern United States.

Mite Webs (#2)

Part of the reason mites are difficult to control is due to the webbing. Mite webs protect eggs and mobile mites from contact by sprays. Webbing is usually confined to the main interveinal areas of the underside of the leaf.

Mite Damage (#3)

Mites damage hop leaves by inserting needle-like mouth parts into plant cells and pumping out the contents. Injury appears as light colored spotting. Such feeding injury is an early clue in detecting an infestation of mites.

Control options vary with the intensity of the infestation. Removal by hand, pulling the leaves off and destroying them, or using a hose to knock them off with water are the least invasive measures. Or, you can use deterrents like pepper sprays or garlic sprays. Planting borders of flowers such as African marigold, nasturtiums and garlic plants will help deter the numbers in the early stages of growth. Organic insecticides such as Pyrethrum, insecticidal soap, nicotine and diatomaceous earth work well if effectively applied. Some success can be derived via the introduction of ladybugs and lacewing predator insects as long as the predators decide to stay on the hops. But if the infestation has gone unchecked, when discovered, using commercial insecticides such as diazinon or malathion sprays may be your only option to try and save the crop.

An ounce of prevention is worth a pound of hops.

DISEASE SYMPTOMS

Powdery Mildew (sphaerotheca macularis)

The fungus first appears as small, circular, powdery colonies on the leaves of hops (see photo). Young and rapidly growing tissue is very susceptible to infection by HPM. As the disease progresses, the colony size increases from about a sixteenth of an inch, to a half-inch or larger. More colonies appear on the leaf surface and may eventually turn the leaf and occasionally the entire vine white. Infection of the female flower (or burr) is the most serious form of the disease. If the burr is infected early in its growth, the female flower or cone may not develop at all or may become deformed and brittle. For susceptible varieties under severe disease pressure, yields can drop by 80% or more, even with the application of fungicides.

Small white colonies of HPM on the commercial variety Galena mark the beginning of infection (inside the three circles). These colonies will enlarge and produce more spores that can spread to other young leaves and cones and eventually turn entire leaves white with their powdery mass.

DISEASE SYMPTOMS

Downy Mildew (pseudoperonspora humuli)

Under favorable weather conditions, the mildew spreads from these shoots and infects leaves and other shoots. In the summer, the leaves of infected vines often turn yellow and the vine may die. Other vines may be dark green and leaves cupped and arms stunted. Cone blight often occurs if favorable moisture conditions are present. Infects both above ground and below ground portions of the hop plant.

(#1)

The root and crown tissue will have reddish-brown flecks and streaks. This should not be confused with normal reddish tissue found in the center of hop roots and crowns.

(#2)

In the spring, infected shoots will appear with normal shoots, growing from a crown.

(#3)

Infected tissue will often develop a black rot.

(#4)

The leaves appear yellowish green, stunted and are cupped downward.

See the Downy Mildew entry in the PNW Plant Disease Management Handbook for more on this disease.

Control Options for the Mildews

The use of Cocide, a copper-based fungicide, and sulfur are two possible solutions to controlling Powdery & Downy mildews.

Black Root Rot: (Phytophthora citricola)

Develops from standing water due to poor drainage or just plain over-watering. Symptoms are general decline in bine growth and wilting, blackened and softening of the root tissue.

For further discussion, see the PNW Plant Disease Management Handbook's entry for Black Root Rot.

Viruses

Symptoms become particularly common during cool periods following a period that has been favorable for hop growth.

Yellow ring spots and line patterns develop on the leaves. As the disease progresses, the yellow areas die and may involve the whole leaf.

On some varieties, the leaves roll upward with considerable interveinal necrosis.

The side arms are shunted and vine growth is poor. Quite often, the growing tip of the vine curves downward and becomes brittle and dies. As the new shoots grow, this also happens to them. The leaves are dark green and curled downward. The cones may also turn brown and fail to develop.

A more detailed overview can be found on the PNW Plant Disease Management Handbook page: Virus Diseases.